Nitrogen-Filled Hollow-Core Fiber Shifts Ultrafast Laser Spectra toward the Infrared

Researchers from Institut National de la Recherche Scientifique (INRS) in Quebec City have introduced a method for tuning a laser’s spectrum to the infrared range. In collaboration with international partners, a team led by Luca Razzari used a hollow-core fiber filled with nitrogen to deliver optical pulses shorter than those delivered by the input laser, and with a high spatial quality.

Existing ultrafast laser technologies are not, or not easily tunable, in the infrared range, requiring nonlinear processes, stages, and/or components to shift emission wavelengths.

Commonly, hollow-core fibers contain monatomic gases, such as argon, to symmetrically broaden a laser spectra and then recompress them into compressed optical pulses. The researchers demonstrated that although a nitrogen-filled capillary fiber is able to broaden the spectrum of a laser, the spectrum in this instance shifted toward less energetic infrared wavelengths. The reaction is a nonlinear response caused by the rotation of gas molecules, meaning the scientists could easily control gas pressure in the fiber.

A laser optical pulse (blue) enters from the left into the hollow-core fiber filled with nitrogen gas (red molecules) and, along propagation, experiences a spectral broadening toward longer wavelengths, depicted as an orange output beam (right). This nonlinear phenomenon is caused by the Raman effect associated with the rotations of the gas molecules under the laser field, as schematically illustrated in the bottom panel. Courtesy of Riccardo Piccoli (INRS).

After the beam broadened (moved toward the infrared), the researchers filtered the output spectrum to preserve on the band that it needed. Energy in the approach transfers into the near-infrared spectral range with comparable efficiency to that of the optical parametric amplifier (OPA) in a pulse that is three times shorter than the input.

The OPA is an established tool that is able to move to the infrared window, and OPA systems are additionally broadly tunable. OPA systems are complex, though, often consisting of multiple, distinct stages.

The new method requires neither external apparatuses nor an additional pulse post-compression system to achieve functionality.

Similar research, led by Vienna-based scientists Andrius Baltuska and Paolo Carpeggiani, used a process that compressed the hollow-core fiber with mirrors, which served to adjust the phase of the broadened pulse, instead of the spectrum filtering process. Though the shift in the infrared diminished in the system and was not as extreme as in the Canadian team’s demonstration, the resulting pulse was much shorter. The intensity of the new pulse made it suitable for attosecond and strong-field physics, Carpeggiani said.

After discovering the similarities of their phenomena, the two teams pooled their expertise. A third team, led by Aleksei Zheltikov and based in Moscow, then developed a theoretical model aimed at explaining the results.

The three-part team believes the new method can help meet demand requirements for long-wavelength ultrashort sources in both laser and strong-field applications. Strong-field physics applications, including those that are laser-based, include high-order harmonic generation, photoelectron spectroscopy, plasma physics, and micromachining. The team identified industrial-grade tunable systems based on ytterbium laser technology as another area of application.

The Natural Sciences and Engineering Research Council of Canada (NSERC); Prompt, a Montreal-based nonprofit organization dedicated to facilitating partnerships and R&D financing between businesses and the public research sector; the Austrian Science Fund (FWF); the Russian Foundation for Basic Research (RFBR); the Welch Foundation; and the Russian Science Foundation supported the research.

The research was published in Optica (www.doi.org/10.1364/OPTICA.397685).